Composite coating for the superalloys

ABSTRACT

Improved operating lifetimes are provided for the superalloys through use of a composite coating comprising a chromium or chromium-rich interlayer adjacent the superalloy substrate surface and an oxidation-resistant outer layer comprising an alloy of iron, cobalt and/or nickel alloyed with selected amounts of chromium, aluminum and yttrium.

United States Patent Simmons, Jr.

[45] Mar. 14, 1972 [54] COMPOSITE COATING FOR THE 3,041,040 6/1962Levinstein ....29/ 198 SUPERALLOYS 2,861,327 11/1958 Bechtold ....29/19s3,552,953 l/197l Lemkey... ....75/l7l [721 lnvemorl Alfred slmmons, EastHartford 3,215,512 11/1965 Coad ..29/191 Conn.

[73] Assignee: United Aircraft Corporation, East Hart- Primary Emfninerflyland Bizot ford, Conn. Attorney-Richard N. James [22] Filed: NOV. 17,196 [57] ABSTRACT [21] Appl. No.: 877,321

Improved operating lifetimes are provided for the superalloys [52] U.S.Cl. ..29/194, 29/1966, 29/198 hr gh use of a omp ite coa ing comprisinga chromium or [51] Int. Cl ..B32b 15/00 chromium-rich interlayeradjacent the superalloy substrate [58] Field of Search ..29/ 198, 194,196.6; 75/171 surface and an oxidation-resistant outer layer comprisingan alloy of iron, cobalt and/or nickel alloyed with selected [56]References Cited amounts of chromium, aluminum and yttrium.

UNlTED STATES PATENTS 4 Claims, 1 Drawing Figure 2,993,264 7/1961Grenoble ..29/198 KVPPf/l/T f/V/Nf fifQV/Pf/Wf/VZJ l \1 Q 1% Q \sCOMPOSITE COATING FOR THE SUPERALLOYS BACKGROUND OF THE INVENTION Thepresent invention relates in general to high-temperature,oxidation-resistant coatings for the superalloys, particularly asapplied to gas turbine engine components.

A limiting factor in the application of many of the superalloys todemanding environments such as those encountered by jet engine hardwareis their susceptibility to high-temperature oxidation and corrosion. Forthis reason these alloys are generally provided with suitable surfacecoatings for increased oxidation resistance. For current operatingconditions the most widely used coatings have been provided by reactingaluminum with the alloy to form surface aluminides which preferentiallyoxidize to form surface oxides through which the transport rates of theoxidizing species are low. Typical of processes of this type is thatdescribed in the US. Pat. No. to Joseph 3,102,044.

Both turbine blade and vane life in existing engines, and the extent ofpower increases requiring higher engine operating temperatures, arelargely limited by the durability of the coatings. In the past, theinadequacy of current coatings to give long term protection againstcorrosion at very high temperatures has prevented use of some of thestronger nickelbase alloys, such as B-l900, in applications where theirproperties otherwise indicate the desirability of their use.

At high temperatures in the dynamic oxidizing environment of a gasturbine engine, temperature fluctuations caused by the mixing of hotcombustion gases with cooler secondary air, or those associated withvariations in engine power levels, give rise to thermally inducedstrains in the coatings at the metaloxide interface which aresufficiently large to spall the protective oxide layer. Furthermore, ata temperature of about 2,000 F nickel and the nickel-base superalloysbegin to exhibit a great alloying affinity for the usual coatingconstituents, and particularly for aluminum, as recognized in the US.Pat. No. to Maxwell 3,450,212. Thus, a loss of coating protection in adynamic oxidizing environment at very high temperature, involves both aninward and an outward loss of one or more of the protective species.

In a series of copending applications of the present assig'nee, thereare described a number of coating compositions for the superalloys whichhave doubled the endurance of the coated components at high temperatureand have in addition permitted engine performance increases associatedwith the higher temperatures of current interest. In application Ser.No. 731,650, filed May 23, 1968 for an Iron Base Coating for theSuperalloys, now US, Pat. No. 3,542,530 there is described a preferredcoating alloy comprising, by weight, 25-29 percent chromium, 12-14percent aluminum, 0.6-0.9 percent yttrium, balance iron, hereinafterreferred to as the FeCrAlY coating. In application Ser. No. 795,616filed Jan. 31, 1969 for a Cobalt Base Coating for the Superalloys, thereis described a preferred coating composition comprising, by weight,19-24 percent chromium, 13-17 percent aluminum, 0.6-0.9 percent yttrium,balance cobalt, hereinafter referred to as the CoCrAlY coating. ANiCrAlY coating comprising, by weight, 20-35 percent chromium, 15-20percent aluminum, 0.050.3 percent yttrium, balance nickel is disclosedin application Ser. No. 734,740 filed June 5, 1968. All of the abovecoating alloys are resistant of oxidation, thermal spalling, and tointerdifiusion with the substrate when compared to alternative coatingschemes. However, it has been found that even with these advancedcoatings there exists a measure of coating-substrate interdiffusion.

It is known that, in some instances, improved coating performance may beobtained through coating processes involving multiple surfacetreatments. In the U.S. Pat. No. to Gibson 2,809,127, the surface of analloy is first chromized and then aluminized to increase the oxidationresistance at high temperature. As in the case of Joseph, supra, thebasic oxidation protection in Gibson is dependent upon the reaction ofaluminum with the constituents of the substrate at the surface to beprotected.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide an improved coating for the superalloys characterized by longterm durability in dynamic oxidizing environments at very hightemperatures. There is provided a composite coating comprising achromium or predominantly chromium interlayer at the superalloy surfaceto be protected and an outer layer of highoxidation resistancecomprising an alloy of iron, cobalt or nickel containing selectedamounts of chromium, aluminum and a rare earth element such as yttrium.

In a preferred embodiment of the invention, the composite coatingcomprises an interlayer of chromium and an outer layer consistingessentially of, by weight, 25-29 percent chromium, 12-14 percentaluminum, 0.60.9 percent yttrium, balance iron.

In another preferred embodiment, the composite coating comprises aninterlayer of chromium and an outer layer consisting essentially of, byweight, 19-24 percent chromium, 13-17 percent aluminum, 0.60.9 percentyttrium, balance cobalt.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a chart comparing thevarious coatings for the nickel-base superalloys in terms of durability.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the generation of theFeCrAlY, CoCrAlY and NiCrAlY coating alloys, and as currently providedin production jet engines, component surface protection has nonnallybeen provided by exposing the substrate to aluminum or aluminum vapor athigh temperature and promoting a reaction of the aluminum with one ormore of the substrate constituents to form protective aluminides. In theFeCrAlY-type coating system, the oxidation protection is effected, notby a coatingsubstrate reaction, but rather by the coating alloy per se.The coating alloy of itself is oxidation-resistant and relatively immuneto thermal spalling and no intermediate coatings are required in termsof the basic function which the coating is to provide, nor in fact isany interdiffusion of substrate or intermediate layer constituents intothe coating desired. In the present composite coating, an interlayer ofchromium is provided to specifically reduce the outer coating-substrateinterdifiusion and by so doing to improve the durability of the coatingas demonstrated by an increased operating lifetime for a component socoated.

Thus, the durability of the FeCrAlY-type coatings have been found to belimited not by deficiencies in the oxidationerosion resistance of thecoatings per se, but rather is a function of the extent of aluminumdepletion in the coating resultant from the coating-substrateinterdifi'usion, particularly at temperatures in excess of about 2,000F.

It was found that a substantial improvement in the endurance of theFeCrAlY-type coatings can be provided by interposing an interlayer ofchromium or a predominantly chromium alloy between the outer coating andthe substrate to act as a diffusion barrier therebetween, minimizing thedepletion of aluminum in the outer coating by this mechanism. Thischromium interlayer may be produced by any of the available methods forgenerating such coatings or surface layers including electroplating,electroplating plus diffusion heat treatment, pack cementation, plasmaspray, slurry spray, or any other technique providing a predominantlychromium layer at or on the substrate surface. It is relativelyimmaterial how the interlayer formed subject, however, to therequirement that the process be one yielding an interlayer composedprimarily of chromium.

The FeCrAlY-type outer coatings are typically applied utilizing vacuumvapor deposition methods and apparatus. As explained, the efficacy ofthese coatings is dependent upon the correct coating alloy compositionbeing deposited on the surface to be protected. These coatings arecharacterized by high-melting points as alloyed and by diverse meltingpoints insofar as the elemental constituents are concerned. Care musttaken in the coating formation process to provide all of the desiredcoating alloy species in the correct proportions in the coating asapplied. Satisfactory results have been attained by vapor deposition ina vacuum utilizing an electron beam heat source, as suggested in theU.S. Pat. No. to Steigerwald 2,746,420.

It should be noted that it is the unique combination comprising thecomposite coating that provides the coating endurance improvementsestablished by test. One of the incidents of the undesirablecoating-substrate interdiffusion, in addition to aluminum depletion inthe coating, is contamination of the substrate by the coatingconstituents. The use of the chromium interlayer has been found not onlyto prevent such detrimental contamination by the coating elements butalso to provide none of itself. In addition, the chromium interlayeradjacent the FeCrAlY coating has appeared to provide no observabledetrimental effect on the coating alloy itself nor on its adherence tothe substrate.

Tests conducted on several nickel-base superalloy substrates, includingsuch superalloys as B4900, MAR M200, and NX 188, and on the cobalt-basesuperalloys such as MAR M302, have indicated that coating lifeimprovements on the order of 50 percent are achieved, as graphicallyillustrated in the drawing.

EXAMPLE Various nickel-base and cobalt-base superalloy parts to becoated were embedded in a pack of blended powders composed of, byweight, 84.5 percent alumina, 15 percent chromium, and 0.5 percentammonium chloride. After purging with argon, the pack was sealed and theparts were chromized at 2,l F. for 4 hours. In general, surface buildupsof 0002-0005 in. resulted from pack chromizing under these conditions.

Subsequent to the chromizing operation, parts were mounted in the vacuumchamber of electron beam melting apparatus, preheated, and coated byvapor deposition from a molten pool of coating material in a vacuum ofTorr or better to typical outer coating thicknesses of 0001-0005 in.

Following deposition of the outer coating, the coated cobalt-basesubstrates were heat treated at 1,900 F. for about an hour in vacuumwith a cool in a nonoxidizing atmosphere at a rate equivalent to aircool. The nickel-base superalloy substrates after coating, and thecobalt-base superalloy substrates after coating and heat treatment, ascoated, were dry glass bead peened at N for about 2 minutes inaccordance with AMS 2,430. Subsequent to peening the coated parts wereheated to 1,975 F. in dry argon or hydrogen, or vacuum; held at heat for4 hours; and cooled at a rate equivalent to air coolmg.

A variety of superalloy substrates were provided with several compositecoating combinations, particularly with respect to the outer coatingcomposition. After extensive testing, it was determined that thepreferred FeCrAlY outer coating chemistry conformed to the following:

Component percent by weight chromium 25-29 aluminum 10.5-12.5 yttrium0.4-0.9

oxygen 0.03 max. nitrogen 0.01 max. hydrogen 0.01 max.

0.5 max.

remainder other elements, total iron The most preferred CoCrAlY coatingin the composite coating consisted of:

broader aspects it is not limited to the exact details described, forobvious modifications will occur to those skilled in the art.

What is claimed is:

l. A composite article resistant to oxidation at high temperaturecomprising:

a substrate selected from the group consisting of the hightemperaturenickel-base and cobalt-base alloys having strengths suitable forstructural applications in a gas turbine engine environment,

an interlayer, adjacent the substrate surface and bonded thereto,selected from the group consisting of chromium and its alloys,

and an oxidation resistant outer layer thereover, bonded to theinterlayer, which consists essentially of chromium, aluminum, at leastone rare earth element, and at least one element selected from the groupconsisting of iron, cobalt, and nickel.

2. A composite article according to claim 1 wherein: in the outer layer,

the chromium content is 1 5-30 weight percent,

the aluminum content is l0-20 weight percent,

the rare earth element is yttrium,

and the yttrium content is at least 0.1 weight percent.

3. A coated gas turbine engine component comprising:

a substrate selected from the group consisting of the high temperature,high-strength nickel-base and cobalt-base alloys,

an interlayer, adjacent the substrate surface and bonded thereto,selected from the group consisting of chromium and its alloys,

and an oxidation resistant outer layer superimposed on and bonded to theinterlayer, the outer layer consisting essentially of, by weight, 25-29percent chromium, 10-14 percent aluminum, 0.4-0.9 percent yttrium,balance substantially iron.

4. A coated gas turbine engine component comprising:

a substrate selected from the group consisting of the high temperature,high-strength nickel-base and cobalt-base alloys,

an interlayer, adjacent the substrate surface and bonded thereto,selected from the group consisting of chromium and its alloys,

and an oxidation resistant outer layer superimposed on and bonded to theinterlayer, the outer layer consisting essentially of, by weight, 21-25percent chromium, 10-1 5 percent aluminum, 0.4-0.9 percent yttrium,balance substantially cobalt.

2. A composite article according to claim 1 wherein: in the outer layer,the chromium content is 15-30 weight percent, the aluminum content is10-20 weight percent, the rare earth element is yttrium, and the yttriumcontent is at least 0.1 weight percent.
 3. A coated gas turbine enginecomponent comprising: a substrate selected from the group consisting ofthe high temperature, high-strength nickel-base and cobalt-base alloys,an interlayer, adjacent the substrate surface and bonded thereto,selected from the group consisting of chromium and its alloys, and anoxidation resistant outer layer superimposed on and bonded to theinterlayer, the outer layer consisting essentially of, by weight, 25-29percent chromium, 10-14 percent aluminum, 0.4-0.9 percent yttrium,balance substantially iron.
 4. A coated gas turbine engine componentcomprising: a substrate selected from the group consisting of the hightemperature, high-strength nickel-base and cobalt-base alloys, aninterlayer, adjacent the substrate surface and bonded thereto, selectedfrom the group consisting of chromium and its alloys, and an oxidationresistant outer layer superimposed on and bonded to the interlayer, theouter layer consisting essentially of, by weight, 21-25 percentchromium, 10-15 percent aluminum, 0.4-0.9 percent yttrium, balancesubstantially cobalt.